A conventional refrigeration system may include a rack of multiple compressors connected to several refrigeration circuits. A refrigeration circuit is defined generally as a physically plumbed series of cases operating at the generally same pressure and/or temperature. For example, in a grocery store, separate refrigeration circuits may exist for frozen food, meats and dairy, with each circuit having one or more cases operating at similar temperature ranges, and the circuits operating in different temperature ranges. The temperature differences between the circuits are typically achieved by using mechanical evaporator pressure regulators (EPR) valves or other valves located in series with each circuit. Each EPR valve regulates the pressure for all the cases in a given circuit. The pressure at which the EPR valve controls the circuit is typically set during system installation, or recalibrated during maintenance, using a mechanical pilot screw disposed in the valve. The circuit pressure is selected based on a pressure drop between the cases on the circuit, the rack suction pressure, and case temperature requirements.
The multiple compressors are connected in parallel using a common suction header and a common discharge header to form a compressor rack. The suction pressure for the compressor rack is determined by modulating each of the compressors between an ON state and an OFF state in a controlled fashion. The suction pressure set point for the compressor rack is generally set to a value that can meet the lowest evaporator circuit requirement. In other words, the circuit that operates at the lowest temperature generally controls the suction pressure set point, which is fixed to meet the refrigeration capacity requirements of that lowest temperature.
Case temperature requirements generally change throughout the year due to ever-changing outside temperature conditions. For example, in the winter, there is generally a lower case load, which may require a higher suction pressure set point. Conversely, in the summer, there is generally a higher load, which may require a lower suction pressure set point. Cost savings from efficiency gains may be realized by seasonally adjusting EPR valves to tailor the output of the refrigeration system to that which is required to meet the seasonal case load. By changing the EPR valves, the suction pressure set point of the compressor rack is adjusted to effect refrigeration system output. Because adjustments to the EPR valves typically require a refrigeration technician, such adjustments are seldom performed on-site due to cost and time constraints.
Electronic EPR valves, such as those disclosed in Assignee's U.S. Pat. Nos. 6,360,553; 6,449,968; 6,601,398; and 6,578,374, each of which is incorporated herein by reference, do not suffer from the above-mentioned disadvantages. The EPR valves provide adaptive adjustment of the evaporator pressure for each circuit, resulting in a more accurate and stable case temperature, but require a separate driver for each EPR valve.